To simplify the heat transfer devices associated with reactors cooled with liquid metal, and to guard against any reaction between the active sodium and water, it is necessary to provide a double separation between the two fluids. For this purpose, it has been proposed to employ steam generators of a special type comprising double-walled heat transfer tubes with an interposition of an exchange fluid which may be liquid, sodium between the two walls of the tube.
It has also been proposed, in the case of double-walled tubes formed by an inner tube and an outer tube which are slid into each other, to fill the small space remaining between the tubes with a neutral gas, such as helium under pressure.
The spaces between the inner and outer tubes forming the double-walled tubes are placed in communication with a leakage control space making it possible to detect, for example by pressure measurement, a possible leak in the wall of one of the tubes during the operation of the steam generator.
The steam generator consists of a bundle of tubes contained in an enclosure, each of the internal tubes of the double-walled tubes communicating at one of its ends with a water distribution system and at its other end with a steam collection system.
The liquid sodium heated in contact with the reactor core is brought into the enclosure of the steam generator, in its upper part, and flows downwards through this enclosure in contact with the outer tubes of the double-walled tubes forming the bundle.
A pressure difference exists between the primary sodium flowing in contact with the outer surface of the tubes and the water or steam circulating inside the tubes of the bundle. The helium filling the leakage control spaces is at an intermediate pressure between these two pressures.
Any leakage through the inner tube or through the outer tube forming the double walled tube is exhibited as a pressure change in the leakage control space.
In prior art devices, described for example in French Pat. Nos. 2,371,655 and 2,379,881, leakage control spaces are provided at each end of the steam generator, close to the water and steam collectors. This makes it necessary to provide double tube plates which require numerous welds in a region which is highly stressed, particularly by thermal stresses.
Moreover, in high power steam generators it is necessary to employ double walled tubes of very great length which may, if appropriate, be wound to form bundles of acceptable length.
It is not possible, in general, to produce double walled tubes of a sufficient length, and it is therefore necessary to join several sections of double walled tubes end to end. This end-to-end joining can, of course, only take place in the case of the inner tube, in a region where the latter is not protected by the peripheral tube. These sections of single tubing inserted between two sections of double walled tubing must be arranged in the steam generator so as not to come into contact with the liquid sodium.
For this purpose, annular chambers which are arranged outside the steam generator enclosure are provided, into which pass the sections of inner tubes permitting the joining of the double walled tubes, through sealed passages in the steam generator enclosure.
Such a device is complex and requires special manufacturing operations which are difficult to carry out.